An operation of ascertaining, in respect of surface area, displacement of a part of or also the entire surface of an object, including a three-dimensional body of any shape, is a task which is essential in the context of many industrially relevant questions. Thus for example partial displacement of the surface of an object occurs in testing components for possible weak points, for the stress analysis of complex systems, and in checking components in regard to complying with certain stress limit values, insofar as, due to a loading, for example a compression loading, a body being observed is elongated slightly at the surface in the region of a weak point in its wall or its layer structure, or a hollow body also experiences complete elongation by virtue of a compression loading.
Known strain measurement methods are either very expensive in terms of application (for example wire strain gauges--only punctiform measurement, surface layer processes), highly inaccurate (brittle lacquer method) or can only be used under particular conditions (cyclic loading in the case of thermoemission analysis).
Speckle interferometry permits contact-less areal detection of displacement and/or deformation on any components. 2D- and 3D-speckle interferometers make it possible to determine deformation in two or three co-ordinate axes. For that purpose the components of the displacement of points of the surface, measured with the speckle interferometer, in one or more directions, for a very large number of points, are converted together into the co-ordinate system of the object or a spatial co-ordinate system. That will be described in greater detail with reference to the Figures.
Thus EP-A0 731 335 already shows such a method of detecting undesirable deformation of an object, which generally occurs under loading, in which speckle interferometry is used employing the specific method of shearography. In that procedure however the configuration of the object is not determined. The apparatus has two separate cameras and a double-armed Mach-Zehnder interferometer.
In addition DE-A-41 02 881 also describes a method of detecting deformation by means of speckle interferometry, in which case the particularity thereof is that the illumination necessary for 3D-deformation is used from different directions with light involving different properties (wavelength, polarisation etc) in order to be able to implement at the same time the recordings which are based on the different illumination directions, which is necessary in particular for detecting deformation on objects whose deformation does not come to a halt or which are to be observed during the deformation.
Knowledge of the object geometry and the relative position with respect to the speckle interferometer are however desirable for accurately determining the three-dimensional displacement vector.
The geometry of the object can either be inputted manually or it can be detected by a measurement procedure using another measuring instrument, for example a fringe projection system. In that case, the surface of a body is radiated with a regular pattern, in particular a simple fringe pattern, which gives a modified pattern on the surface of the body. If for example a ball is irradiated with such a fringe pattern, then the result obtained is lines which are ever increasingly curved outwardly in a lens configuration on the surface of the ball.
On the basis of knowledge of the geometrical data of the irradiation pattern and the spacing relative to the object, it is possible to compute therefrom the shape of the surface of the object.
The disadvantage of that mode of operation is the high level of financial expenditure involved in providing two different expensive measurement systems for displacement measurement and shape measurement.